Position sensing system effective to produce an output signal as function of inductance

ABSTRACT

A sensing or control system, effective to sense the position of a mechanical element and produce an electrical signal in accordance with said position, includes a magnetic member adapted to be moved in response to the movement of the mechanical element into magnetic engagement with a magnetic core received within and extending from an induction coil, thereby to change the inductance of said coil. The signal producing means comprises an electrical circuit incorporating said inductance coil and effective to compare a voltage responsive to the inductance of said coil with a reference voltage and calibrated to null the signal at a desired position of said mechanical element.

produce an electriposition, includes a magin response to the moveagneticengagement ing from an of said coil.

id coil with a gnal at a desired References Cited Leonard g or controlsystem, effective to sense y to change the inductance means comprises anelectrical circuit in- UNITED STATES PATENTS 2,972 07l 2/l96l 3.032.7055/1962 Olsen v Primary Examiner- Lee T. Hix Attorney-James and FranklinABSTRACT: A sensin the position of a mechanical element and cal signalin accordance with said netic member adapted to be moved ment of themechanical element into m with a magnetic core received within andextend induction coil, thereb The signal producing corporating saidinductance coil and effective to compare a voltage responsive to theinductance of sa reference voltage and calibrated to null the siposition of said mechanical element.

42 I 46 ga TION OF 323/90, 336/92 H05 HOlf 27/06 323/5l, 90; 3l7/DlG. 2;340/266, 280, 282

Inventor Arthur M. Cohen Westport. Conn. Appl No 2,993 Filed Jan. 15,I970 Patented July 27, 1971 Electric Regulator Corporation Norwalk,Conn.

PRODUCE AN OUTPUT SIGNAL AS FUNC INDUCTANCE 9 Claims, 3 Drawing Figs.[52] US. CL...

Int. Field of /7 United States Patent [73] Assignee [54] POSITIONSENSING SYSTEM EFFECTIVE T0 POSITION SENSING SYSTEM EFFECTIVE TO PRODUCEAN OUTPUT SIGNAL AS FUNCTION OFINDUCTANCE The present invention relatesto a system for sensing the position of a mechanical element andproducing an output signal in accordance with such position. Devices forconverting mechanical position into an electrical signal, often referredto as transducers, are commonly employed in control systems for thepurpose of controlling: the position of a mechanical element. In controlsystems of this type the signal generated in response to the position ofa mechanical element is generally applied to a control instrumentalityin order to produce the desired result, usually to maintain said elementin a particular position. Sensing systems of this type have taken manyforms in the past. A common form of this device utilizes the electricalcontact of a member or members adapted in response to the movement ofthe controlled element, to move along a resistor to thereby vary theresistance of the latter. An electrical signal is produced which isresponsive to such change in resistance.

Many problems arise in the design and use of systems and devices of thischaracter. Sensitivity is always an important factor. The power handlingcapacity of the device is another important factor usually antitheticalto sensitivity. If large amounts of power are to be handled directly bythe device itself, it must be so constructed as to be inherentlyrelatively insensitive. Closely related to sensitivity is the propertyof continuity. Thus, a sequence of electrical contacts by a movingmember with a resistor will produce a stepped output signal in responseto such movement. The'sensitivity of the device can obviously be nobetter than the electrical magnitude of the step. Reliability anddependability are factors which are also related to sensitivity andpower handling capacity. In general, the more sensitive the device andthe more power it must directly handle, the shorter isits operating lifeand the more susceptible it is to improper operation. A further aspectof sensitivity relates to the magnitude of force which must be exertedon the mechanically moved element in order to give rise to a change inthe control electrical characteristic. Again, this is related to theparticular type of electrical contact utilized.

The usual form of rheostat, or variable resistor, comprises a resistancewire helically wound about a supporting form, a brush being slidablealong the form so as to make contact with different portions of thewinding. With a device of this type uniform or continuous variations ofresistance is not feasible since the movement of the brush from oneoperative position to its next adjacent operative position involves theconnection to, or disconnection from, the circuit of a predeterminedlength of resistance wire having its own inherent'resistance. Moreover,the overall electrical characteristics of devices of this type aredependent upon theelectrical. connection between the brush andwinding.These'elements are particularly sensitive to wear, to the deleteriouseffect of dust and dirt, to oxidizing and pittingof the brush and to theamount of power which is directly handled by the device.

Another type of mechanicoelectrical transducer comprises 'a plurality ofelectrical contact pairs adapted to be sequentially opened and closed inaccordance with the movement of a control element. With a device of thistype, the disadvantages inherent in the use of a sliding brush are notpresent but the electrical output of the device again can be varied onlyin a distinctly stepwise manner so that its sensitivity leaves much tobe desired. A comparatively recent development is the brushlessmechanical-electrical transducer, disclosed in my U. S. Pat. No.3,057,979, which eliminates, to some extent, the disadvantage ofthe-prior art structuresmentioned above. This was achieved by modifyingthe mode of actuation of a contactcontrol member to cause continuoussequential opening and closing of a plurality of pairs of electricalcontacts. This is accomplished. by superimposing a dither action uponthe mechanical positioning of the contact-control member by the externalmechanically moving element. As disclosed in the aforementioned patent,an electromagnetic coil is energized with a suitable altematingcurrent.The electromagnet acts upon a magnetizable element which is operativelyconnected to said contact-controlling member. The dither is sufficientto cause the member to vibrate so as to open and close a given contactpair while the external mechanically moving element remains in a givenposition. Said given position determines the length of time a givencontact pair will be open or closed, thus defining a value of resistancesomewhere intermediate the values defined by a steady state open orclosed position of said given contact pair. The result is a relativelysmooth, accurate output signal sensitive to small changes in theposition of the mechanically moving external clement. However, becausethe device utilizes electrical contacts, some of the aforementionedproblems of wear, dust and dirt, oxidizing, and pitting remain, at leastto some extent, with regard to the contact member. Indeed, the problemof wear becomes even more acute where the contact member opens andcloses l20 times per second (in response to a 60 c.p.s. current appliedto the coil) as does the member to which the dither is applied in myaforementioned patent.

By contrast, the sensing system of the present invention eliminatescompletely the problems associated with electrical contact apparatus byutilizing the effect of a change of an electromagnetic field on theinductance of an induction coil. The resulting change in inductance isutilized as the controlling electrical property.

It is therefore a primary object of the present invention to provide asensing device adapted to vary an electrical property in response to theposition of a moving mechanical element and which is continuouslysensitive to small changes in said position.

It is another object of the present invention to devise an electricalcircuit adapted to produce an output signal which accurately reflectsthe value of such electrical property notwithstanding variations in thesource voltage applied thereto.

To the accomplishment of the above, the device of the present inventioncomprises an inductance coil at least partially surrounded by magneticforming a magnetic circuit for the flux lines generated by said coil. Agap is provided in the magnetic circuit and a magnetizable elementadapted to move in response to movement of an extemal mechanical elementis positioned with respect to the coil and the magnetic circuit so as toprogressively increase or reduce the gap in response to such movement inone direction or the other. The induction coil is operatively connectedto an electrical circuit effective to compare the voltage across thecoil to a reference voltage.

To the accomplishment of the above, and to such other objects as mayhereinafter appear, the present invention relates to a sensing system asdefined in the accompanying claims and as described in thisspecification, taken together with the accompanying drawing, in which:

FIG. 1 is a circuit diagram of a circuit for producing a signal as afunction of the inductance of a variable inductor.

FIG. 2 is a cross-sectional top plan view taken along line 22 of FIG. 3,showing the sensing device of the present invention.

FIG. 3 is a cross-sectional side elevation taken along line -3 of FIG.2, showing the sensing device of the present invention with the movablebridge member in its spring-biased positlOl'l.

As best shown in FIG. 3, the sensing device is mounted on a support 10to which a frame member 14 is attached by suitable means 12. A housing15' is adapted to fit onto frame 14 along peripheral grooves 17. Framemember 14 includes a pair of extension members 14a extending downwardlyfrom support 10 and adapted to support the structure hereinafterdescribed. The device proper of the present invention comprises aninduction coil 16 on either end of which are end plates 18 and 20respectively. As best shown in FIG. 2, end plate l8 has an aperture 19ofa smaller diameter than-coil l6 and centered on the axis thereof.Plate 18 further includes upstanding portions 18a which are connected bysuitablemeans 22 to frame member 14a (FIG. 3). End plate 20 extendssubstantially beyond the periphery of induction coil 16 and is connectedto housing by a bolt and nut arrangement generally designated 24. Endplates 18 and 20 are connected together and thus each clamped tightlyagainst opposite ends of coil 16 by posts 26 and fasteners 28 and 30respectively. Magnetic spacer members in the form of plates 32 withrectangular cutouts 35 are mounted on posts 26 as by being bent ontosuch posts (FIG. 2) and serve as a magnetic connection between endplates 18 and 20. A magnetic core 34 is positioned within coil 16 and issecured at one end of end plate 20 by bolt 36. The other end of core 34extends outwardly from coil 16 and is received within and extendsthrough aperture 19 of end plate 18. Coil 34 is anchored firmly inposition relative to end plate I8 by means of a washer 38 of nonmagneticmaterial. It will now be readily apparent that coil 16 is firmly securedon a framework defining a magnetic circuit consisting of magnetic core34, magnetic end plates I8 and 20and magnetic spacer means 32, washer 38providing the sole gap in the otherwise closed magnetic circuit.

The moveable portion of my device is again best illustrated in FIG. 3. Abridge member 40 is pivotally mounted on end plate 18 by means of a thinresilient strip 42 secured to end plate 18 at one end and securedbetween bridge member 40 and a support member 46 by bolts 48 at itsother end. An arm 50 adapted to be operatively connected to themechanical ele ment, the position of which is to be sensed, is securedto bridge member 40 by means of another thin strip 52 of resilientmaterial secured at one end to arm 50 by bolt 54 and secured at itsother end between bridge member 40 and support member 56 by bolts 58.Bridge member 40 is provided with an aperture 60 slightly larger thancore 34 and adapted to receive and surround said core when bridge member40 is pivoted to a position parallel to end plate 18. A stop bar 62 issecured on posts 26 above bridge member 40 and serves to prevent bridgemember 40 from pivoting in a counterclockwise direction beyond apredetermined position. Bridge member 40 is normally spring biased tothis position by tension spring 64 connected between end plate 20 andstrip 42. It will be apparent that bridge member 40 is adapted to movebetween the position illustrated in FIG. 3 (hereinafter referred to asthe open position) and a position in which it is flush with end plate18, but spaced therefrom by the thickness of strip 42 (hereinafterreferred to as the closed position). Leads 66 (shown fragmented in FIG.2) connect the terminals of induction coil 16 to leads mounted onsupport 10 and extending externally therefrom.

When induction coil 16 is energized an electromagnetic field isestablished around the coil, as is well known in the art. The resultingmagnetic field tends to urge bridge member 40 toward coil 16. However,tension spring 64 is sufficient to maintain bridge member 40 in theposition illustrated notwithstanding this slight magnetic pull. Themagnetic flux generated by coil 16 passes downwardly through core 34,through end plate 20 and thence through the magnetic spacers to endplate 18. The flux then tends to pass from the interior of aperture 19in end plate 18 back to core 34. Washer 38, being of nonmagneticmaterial, acts as a magnetic insulator which inhibits this flow. Most ofthe flux takes the path of least resistance, jumping the small airgapbetween plate 18 and bridge member 40 near its pivot point. From there,the flux jumps the airgap between the interior of the aperture 60 inbridge member 40 and the core 34. Since the effective airgap (e.g., thecombination of the above two gaps) is smaller than the gap provided bywasher 38, bridge member 40 serves as a bridge between end plate 18 andcore 34 by providing the path of least resistance for the magnetic fluxlines generated by coil I 16. As bridgemember 40 pivots in a clockwisedirection as viewed in FIG. 3, the flux distribution will change becauseof the greater proximity of the bridge member 40 to core 34 (at theinterior of aperture 60) and end plate 18 (at its bottom surface), thegreatest change in proximity occurring at its extreme right-hand end. Itshould be noted that bridge member 40 never provides a completely closedmagnetic circuit for the electromagnetic field generated by coil 16. Inthe'lowermost position of arm 50, bridge member 40 is spaced from endplate 18 by a gap corresponding to the thickness of strip 42 of theorder of 0.01 inches and from core 34 by a gap between said core and theinterior of aperture 60, also of the order of 0.01 inches. Furtherdownward movement of arm 50 will produce a flexing of strip 52 withoutmaterially reducing the gap between bridge member 40 and end plate 18.As a result of the change in flux distribution brought about by movementof bridge member 40, the inductance of coil 16 changes, inductance beinga function of the permeability of the material surrounding'the coil.This change in inductance may be utilized to produce an electricalsignal as a function of the movement' of arm 50.

. A circuit for producing such a signal is illustrated in FIG. 1. Coil16, illustrated schematically as a variable inductor, is placed inseries with a resistor R,, across an alternating voltage source V. Theprimary winding of a transformer T is connected in parallel withinductor 16 and resistor R, across the same voltage source. TransformerT has a turns ratio of one to one and thus the voltage taken off thesecondary winding is equal in magnitude to the voltage source V. A pairof resistors R and R are connected across the terminals of the secondarywinding of transformer T to form a closed current loop. The voltageacross R, is compared with the voltage across R;, by picking the voltageoff node a (between inductor 16 and resistor R, and node b (betweenresistors R and R and connecting said picked off voltages as inputs to apair of full wave bridge rectifiers, generally designated B, the otherinputs being connected to ground at nodes c and d. An output signed Eacross the output terminals of full wave bridge rectifiers B is used forcontrol purposes, e.g., to control the position of the mechanicalelement connected to arm 50. It will be apparent that the voltage acrossresistor R,, picked off at node a, will be equal to the source voltage Vminus the voltage across the coil 16, which voltage will vary as theinductance of coil 16 varies. The voltage across R picked off at node b,will be directly proportional to the source voltage V Inamely,R3/(R2-l-R3) V].

It will be apparent that the output signal E measures the voltage acrosscoil I6 against a reference voltage. Since both voltages are responsiveto the same voltage source V, the output signal E will be insensitive toline variations in the voltage source. The values of the three resistorsR,, R, and R may, of course, be chosen so as to null output signal E atany desired value of the voltage across coil 16, corresponding to agiven position of bridge member 40 and thus, through arm 50, a givenposition of the mechanical element in question. The output signal may beused either to monitor or control the position of such mechanicalelement.

An example of a control system in which the present invention has beenfound particularly useful is the control of thrr ad tension in threadwinding systems. In such a system arm 50 is operatively connected to aslack roller resting on a slack loop joined between a set of supportrollers. As the slack is increased or decreased beyond its desiredvalue, the slack roller changes position, thereby causing bridge member40 to pivot. The resulting change of inductance is sensed by the circuitof FIG. 1 and the output signal is applied to an electric motor which inturn controls the rate feed of the thread. The circuit resistors arechosen so as to null the signal at the desired position of the slackroller. The electric motor is either speeded up or slowed down dependingupon the sign of the output signal which in turn depends on whether theinductance of coil 16 is increased or decreased beyond the nullingpoint.

The sensing system of the present invention provides a means of quicklyand accurately sensing the position of a mechanical element andproducing an output signal in accordance with such position. Because theapparatus utilizes no electrical contact members, the responseencounters none of the disturbances inherent in the making and breakingof electrical contacts. Mechanical movement is converted smoothly andaccurately into an electrical signal by virtue of variations in theintensity of magnetic field. The control signal generated by the circuitherein disclosed likewise accurately reflects the sensed position freefrom line variations in the external voltage source utilized.

While only a single embodiment of this invention has been hereinspecifically disclosed, it will be apparent that variations may be madethereto without departing from the spirit and scope of the invention asdefined in the appended claims.

lclaim:

l. A sensing or control system for sensing the position of a mechanicalelement and producing an output signal as a function of said position,comprising an inductor mounted on a frame and having a core of magneticmaterial positioned therein and extending from one end of said inductor,a member adapted to be operatively connected to said mechanical elementand movable therewith, said member being provided with an apertureadapted to receive the extended portion of said magnetic core andconstructed, at least in the vicinity of said aperture, of magneticmaterial, said member positioned in relation to said core such that uponrelative movement of said member, said member moves toward said inductorand said aperture moves toward a position surrounding and receiving saidcore, thereby changing the inductance of said inductor, and anelectrical circuit operatively connected to said inductor and effectiveto produce an output signal as a function of the inductance of saidinductor.

2. The sensing system of claim 1, further comprising a magnetic spaceradjacent to,but spaced from said inductor and effective to capture themagnetic flux emanating from said extended end of said magnetic core anddirect it back to the other end of said core in a complete magneticcircuit and wherein said member, upon being moved toward said inductor,is effective to capture said magnetic flux and direct it from said coreto said spacer, thereby reducing the effective airgap between theextended end of said core and said spacer, whereby the inductance ofsaid inductor is changed.

3. The sensing system of claim 1, wherein said member is pivotallyconnected to said frame along an axis on one side of said aperturewhereby, upon movement of said mechanical element, said member pivots onsaid frame.

4. The sensing system of claim 3, wherein said pivot axis is insubstantially the same plane as said one end of said inductor, wherebysaid member is adapted to pivot from a position inclined to the plane ofsaid one end of said inductor to a position parallel to the plane ofsaid one end of said inductor and contiguous thereto.

5. The sensing system of claim 4, wherein said member is connected tosaid element on one side of said pivot axis and is connected on theother side of said pivot axis to a resilient member effective to biassaid member away from said inductor.

6. A sensing system for sensing the position ofa mechanical element andproducing an output signal as a function of said position, comprising aframe, an induction coil mounted on said frame and adapted, whenactuated by an electrical current passed therethrough, to develop anelectromagnetic field, a magnetic core positioned within said inductioncoil and extending therefrom at least at one end thereof, a firstmagnetic end plate having an aperture therein and positioned at said oneend of said coil such that said aperture surrounds and is spaced fromthe extended portion of said magnetic core, a second magnetic end platepositioned at the other end of said magnetic coil and operativelymagnetically connected to said core, magnetic spacer means spaced fromsaid coil and operatively magnetically connecting said first and secondmagnetic end plates, whereby when said bridge member is in said firstposition, the magnetic flux lines of said electromagnetic field areadapted to flow in a eireuital field successively through said core,said second end plate, said spacer means, said first end plate, acrosssaid space to said bridge member and thence across a gap defined betweensaid bridge aperture and said core, back to said core, and a magneticbridge member operatively connected to said mechanical element andmovable therewith, said bridge member having an aperture smaller thansaid first end plate aperture, but larger than said core and beingpivotally mounted on said frame, said bridge member adapted to move froma first position at said one end of said coil spaced from said first endplate to a position substantially contiguous to said first end platewith said bridge aperture receiving and surrounding said extendedportion of said core, whereby upon relative movement of said mechanicalelement said bridge member pivots on said frame, with said aperturemoving toward said second position, to capture said flux lines therebytending to reduce said space and said gap to change the inductance ofsaid induction coil, and an electrical circuit operatively connected tosaid induction coil.

7. The sensing system of claim 1, wherein said electrical circuitcomprises a resistor and means for producing a reference voltage, saidinductor being connected in series with said resistor, said signalproducing means comprising means to compare the voltage across saidresistor with said reference voltage.

8. The sensing system of claim 7, wherein said reference voltage isresponsive to the voltage applied across said resistor and saidinductor.

9. The sensing system of claim 8 comprising transformer means connectedin parallel with said inductor and said resistor.

1. A sensing or control system for sensing the position of a mechanicalelement and producing an output signal as a function of said position,comprising an inductor mounted on a frame and having a core of magneticmaterial positioned therein and extending from one end of said inductor,a member adapted to be operatively connected to said mechanical elementand movable therewith, said member being provided with an apertureadapted to receive the extended portion of said magnetic core andconstructed, at least in the vicinity of said aperture, of magneticmaterial, said member positioned in relation to said core such that uponrelative movement of said member, said member moves toward said inductorand said aperture moves toward a position surrounding and receiving saidcore, thereby changing the inductance of said inductor, and anelectrical circuit operatively connected to said inductor and effectiveto produce an output signal as a function of the inductance of saidinductor.
 2. The sensing system of claim 1, further comprising amagnetic spacer adjacent to, but spaced from said inductor and effectiveto capture the magnetic flux emanating from said extended end of saidmagnetic core and direct it back to the other end of said core in acomplete magnetic circuit and wherein said member, upon being movedtoward said inductor, is effective to capture said magnetic flux anddirect it from said core to said spacer, thereby reducing the effectiveairgap between the extended end of said core and said spacer, wherebythe inductance of said inductor is changed.
 3. The sensing system ofclaim 1, wherein said member is pivotally connected to said frame alongan axis on one side of said aperture whereby, upon movement of saidmechanical element, said member pivots on said frame.
 4. The sensingsystem of claim 3, wherein said pivot axis is in substantially the sameplane as said one end of said inductor, whereby said member is adaptedto pivot from a position inclined to the plane of said one end of saidinductor to a position parallel to the plane of said one end of saidinductor and contiguous thereto.
 5. The sensing system of claim 4,wherein said member is connected to said element on one side of saidpivot axis and is connected on the other side of said pivot axis to aresilient member effective to bias said member away from said inductor.6. A sensing system for sensing the position of a mechanical element andproducing an output signal as a function of said position, comprising aframe, an induction coil mounted on said frame and adapted, whenactuated by an electrical current passed therethrough, to develop anelectromagnetic field, a magnetic core positioned within said inductioncoil and extending therefrom at least at one end thereof, a firstmagnetic end plate having an aperture therein and positioned at said oneend of said coil such that said aperture surrounds and is spaced fromthe extended portion of said magnetic core, a second magnetic end platepositioned at the other end of said magnetic coil and operativelymagnetically connected to said core, magnetic spacer means spaced fromsaid coil and operatively magnetically connecting said first and secondmagnetic end plates, whereby when said bridge member is in said firstposition, the magnetic flux lines of said electromagnetic field areadapted to flow in a circuital field successively through said core,said second end plate, said spacer means, said first end plate, acrosssaid space to said bridge member and thence across a gap defined betweensaid bridge aperture and said core, back to said core, and a magneticbridge member operatively connected to said mechanical element andmovable therewith, said bridge member having an aperture smaller thansaid first end plate aperture, but larger than said core and beingpivotally mounted on said frame, said bridge member adapted to move froma first position at said one end of said coil spaced from said first endplate to a position substantially contiguous to said first end platewith said bridge aperture receiving and surrounding said extendedportion of said core, whereby upon relative movement of said mechanicalelement said bridge member pivots on said frame, with said aperturemoving toward said second position, to capture said flux lines therebytending to reduce said space and said gap to change the inductance ofsaid induction coil, and an electrical circuit operatively connected tosaid induction coil.
 7. The sensing system of claim 1, wherein saidelectrical circuit comprises a resistor and means for producing areference voltage, said inductor being connected in series with saidresistor, said signal producing means comprising means to compare thevoltage across said resistor with said reference voltage.
 8. The sensingsystem of claim 7, wherein said reference voltage is responsive to thevoltage applied across said resistor and said inductor.
 9. The sensingsystem of claim 8 comprising transformer means connecTed in parallelwith said inductor and said resistor.